A lithium ion battery has been widely used as a power supply for a mobile telephone, a notebook computer and the like owing to the advantages thereof including a large capacity, a high voltage and a capability of miniaturization. A lithium ion battery has also been greatly expected in recent years as a power supply for power electronics including an electric vehicle and a hybrid vehicle, and has been developed actively.
A lithium ion battery performs battery charge and discharge through migration of lithium ions between a positive electrode and a negative electrode, and in the negative electrode, a negative electrode active material absorbs lithium ions upon battery charge and discharges lithium ions upon battery discharge.
In general, lithium cobaltate (LiCoO2) has been used as the active material of the positive electrode, and graphite has been used as the negative electrode active material.
However, graphite, which has been widely used as the negative electrode active material, has a theoretical capacity of only 372 mAh/g, and increase of the capacity has been demanded. As an alternative material of the carbonaceous negative electrode active material, metallic materials, such as Si and Sn, which are expected to have a large capacity have been actively investigated,
However, Si and Sn absorb lithium ions through alloying reaction with lithium and undergo large expansion and contraction in volume associated with absorption and discharge of lithium ions.
Accordingly, the sole use of Si or Sn for constituting the negative electrode active material provides a problem that particles of Si or Sn are broken or drop-off from the collector due to the stress caused by the expansion and contraction, which deteriorates the cycling characteristics, i.e., the maintenance of the capacity on repeated battery charge and discharge.
As a countermeasure therefor, Patent Document 1 discloses that Si is alloyed to provide a negative electrode active material having such a structure that a large number of Si nuclei are each surrounded by an Al—Co based alloy matrix phase, by which the expansion and contraction stress of the Si phase is relaxed with the matrix phase, thereby improving the cycling characteristics.
Patent Document 1 also discloses that an alloy melt is quenched to provide an Si based amorphous alloy, which is then heat treated to deposit fine crystalline Si nuclei, thereby providing a negative electrode active material for a lithium secondary battery having a fine structure including the $i nuclei and an alloy matrix formed through phase separation from Si upon quenching solidification.
However, the technique disclosed in Patent Document 1 still has room for improvement as follows.
In the structure having an Si phase surrounded by an Al—Co based alloy matrix phase, the Al alloy has slight Li activity but fails to function sufficiently as an Li diffusion path (the Al alloy substantially does not absorb Li), and the capacity utilization factor with respect to the theoretical capacity of the active material is low to fail to enhance the initial discharge capacity. Furthermore, upon using the Al alloy as the matrix phase, the cycling characteristics may certainly be improved, but there is a difficulty in further enhancement of the cycling characteristics.
It is considered that the aforementioned problems may be caused by the factors shown below.
The Al alloy substantially does not absorb Li as described above, and therefore, in the case where the Al alloy is used as the matrix phase surrounding the Si phase, the expansion of the matrix phase itself at the time of the volume expansion of the Si phase is small and thus becomes broken because the matrix phase cannot endure the expansion stress of the Si phase, resulting in difficulty in further enhancement of the cycling characteristics.
Patent Document 2 discloses a lithium secondary battery having a large capacity and enhanced cycling characteristics, and Patent Document 3 discloses a negative electrode active material for a lithium battery having enhanced cycling characteristics while maintaining a large discharge capacity.
However, Patent Documents 2 and 3 do not disclose a negative electrode active material having a structure containing an Si phase as a nuclei and a matrix composed of an Si—Fe compound phase and an Sn—Cu compound phase crystallized to surround the nuclei.
Patent Document 4 discloses nanosize particles, a negative electrode material for a lithium ion secondary battery that contains the nanosize particles, a negative electrode for a lithium ion secondary battery, a lithium ion secondary battery, and a method for producing the nanosize particles, which intends to provide a negative electrode material for a lithium ion secondary battery that achieves a large capacity and good cycling characteristics.
Patent Document 4 discloses in Example in Table 1 an example of an active material containing an Si—Sn—Cu—Fe quaternary alloy.
However, this active material does not have a structure containing an Si phase as a nuclei and a matrix composed of an Si—Fe compound phase and an Sn—Cu compound phase crystallized to surround the nuclei, and thus is different from the present invention.
Patent Document 1: JP-A-2009-32644
Patent Document 2: JP-A-2006-172777
Patent Document 3:JP-A-2002-124254
Patent Document 4: JP-A-2011-32541